Re: How did Koala Bears evolve

hi Sheena,

I wasn't sure exactly what you wanted to know from your question "how did koala bears evolve" - it's a little bit like saying "why is the number seven"! One answer would be that the koala bear came to exist through the same process that all organisms came to exist - evolution through natural selection. In other words, I don't think there is anything special about koala bears compared to any other organism that helps us explain evolution.

So, if I may, I'll interpret your question in my own way and tell you a few things about the biology of koalas, then we can relate their biology to evolution. As you probably know, they are found only in the eastern states of Australia, where they are quite widespread. The scientific name of the Koala is Phascolarctos cinereus, which means "ash-coloured bear with a pouch".They live only in eucalyptus forests, eat only eucalyptus leaves, are mostly nocturnal (active at night) and spend up to 20 hours a days sleeping. For lots more information, look at this excellent koala factsheet.

Now. koalas are mammals, just like you and me - they give birth to live young, suckle the young with milk and are covered in hair. However, they are rather special mammals, belonging to a large family whose scientific name is the Metatheria. You are probably more familiar with the common name of these animals, which is the marsupials. Marsupials are special mammals because they give birth to very undeveloped young, which then mature inside a pouch. This is different to mammals like ourselves, where the young are nourished in the womb through a structure called the placenta and are born fully developed.

Let me explain a bit about all these scientific names. Living things are classified using a branch of science called taxonomy. You can think of this like a tree - at the bottom is the trunk, which is one big group to which all organisms belong. Then we divide up into branches - so all the animals would be in a big branch, but we could divide that into smaller branches for mammals, reptiles, amphibians and so on...and we could divide the mammal branch into marsupials, non-marsupials...and eventually get to a single species. Try this website to see what I mean. We use the following words to describe the progression from the trunk to the branches: Kingdom, Phylum, Class, Order, Family, Genus, Species. So to fully describe a koala, we say it is an animal (Kingdom Animalia), of the phylum Chordata and it's a mammal (Class Mammalia), of the Order Diprotodontia (marsupials including kangeroos, possums and wallabies). The Family is Phascolarctidae (koalas) and the Genus/Species we have seen already, Phascolarctos cinereus.

This seems complicated! However, it's very useful to help us understand how organisms are related and so to be able to think about evolution. We can see that koalas are quite special - they are the only members of their family. Looking further back though, we see that they are related to other marsupials (they have a pouch) and to other mammals (hair, milk, live young). We also know that marsupials are found on other continents (South America) and mammals are found the world over. Now we can get thinking about evolution.

Evolutionary theory tells us that many millions of years ago, there was a common ancestor of all the mammals that we see today. Over time, the mammals evolved into 3 groups: Eutheria (placental mammals like us), Metatheria (marsupials like the koala) and Prototheria (egg laying mammals like the platypus). We also know that the continents that we see today were once connected in a giant supercontinent, which means that all of these types of mammals might have been found all over the world. However, when the supercontinent broke apart, the animals in places like Australia or South America became isolated in new environments, which led to the evolution of the strange creatures that we find in these places. What then happens is that organisms will tend to spread through the environment and occupy as much of it as possible. We can look at the environment and identify distinct parts of it, which we call niches. So for instance, you might say that the niche occupied by koalas is "the tops of eucalyptus trees".Through the process of natural selection, the ancient ancestor of marsupials has diverged into many kinds of marsupial with different lifestyles for different environments - digging in the ground (wombats), ranging across grasslands (kangeroos) and eating energy-poor leaves at the top of trees (koalas).

I hope this has helped you to think about your question - try some of the scientific names in Google or the MadSci archives - marsupials are always popular topics.